Magnesia-carbon bricks composed mainly of MgO have been in general use as a lining material for converters and stainless steel manufacturing furnaces on account of their good spalling and slag resistance.
They are still subject to slag attack when used in an environment where the refining temperature is higher and secondary combustion prevails. This results in the dissolution of the magnesia clinker in slag, leaving the carbon, such as graphite, in bricks always exposed with the surfaces of the bricks. This accelerates the carbon oxidation and, hence, the rate of corrosion.
A conventional way to protect graphite in magnesia-carbon bricks from oxidation is the addition of an antioxidant of a readily oxidizable metal to the brick constituents.
However, an additional antioxidant alone is not enough to prevent the dissolution and oxidation of decarburization layer resulting from a slag attack in refining furnaces operating at 1700.degree. C. or above.
Concerning the improvement of slag corrosion resistance at high temperatures, Japanese Patent Publication No. Sho 54-8206 discloses that electrofused magnesia is superior to sintered magnesia in resistance to corrosion by molten metal and slag when used as a raw material in refractories for steel and non-ferrous metal melting furnaces, because of its power crystal boundaries where mechanical destruction and chemical corrosion begin.
As a raw material of filler for lining refractories for steel and nonferrous metal melting furnaces, electrofused magnesia is used in the form of a grain having a size of 5 to 3 mm, 3 to 1 mm, or 1 to 0 mm, obtained by crushing clinker ingots, which are subjected to classification and particle size regulation.
For electrofused clinker to be a strong filler for refractories, the electrofused magnesia should include a high ratio of single crystal. Clinker ingot with a large ratio of single crystal can be obtained by melting from high-purity magnesia containing more than 99.9 wt. % of MgO. However, this kind of high-purity magnesia is too expensive to be used as a refractory material, so its use is limited to optical windows and electronic device substrates.
A possible means to obtain electrofused magnesia with a high ratio of single crystal is by the electrofusion of a relatively less pure raw material containing 99.0 to 99.8 wt. % of magnesia to obtain clinker ingots from which large crystals are selectively chosen. However, this method is deemed impractical because of low yields.
Meanwhile, it is known that electrofused magnesia used as a filler of refractories exhibits good corrosion resistance because CaO crystallizes out at the grain boundary of the periclase crystal having a high CaO/SiO.sub.2 ratio and reacts with slag to increase its viscosity, thereby preventing its infiltration into the grain boundary.
However, electrofused magnesia with a high CaO/SiO.sub.2 ratio has the disadvantage of limiting the growth of single crystal by increased impurities. Moreover, the addition of CaO to electrofused magnesia poses a problem associated with the slaking of CaO which deteriorates the structure of refractories.
It is an object of the present invention to provide electrofused magnesia clinker having a high ratio of single crystal which is obtained from a raw material of comparatively low purity which has never before been thought to be usable as well as a refractory material of electrofused magnesia having a high CaO/SiO.sub.2 ratio.
It is another object of the present invention to provide a refractory material containing electrofused magnesia clinker which is stable against slag attack and especially has high corrosion resistance against slag with a high iron content, even in an oxidative environment at high temperatures.
It is yet another object of the present invention to provide a refractory material of electrofused magnesia clinker which poses no problems associated with the slaking of CaO when the electrofused magnesia material contains CaO for improving corrosion resistance through the ability of CaO to prevent the infiltration of slag into the grain boundary.
It is a further object of the present invention to provide a burned or unburned refractory brick or a prepared unshaped refractory of good Corrosion and spalling resistance which is produced from highly corrosion-resistant electrofused magnesia clinker.